In recent years, there has been a tendency towards power systems having more and smaller energy sources providing input to the power networks. The focus on climate change and the consequential focus on reduction of CO2 emissions lead away from large coal fired power generators providing a significant share of the total input to the power system, and towards power systems where the share of power from renewable energy sources, such as power from wind, water or solar energy sources, is significantly higher than hitherto. However, renewable energy sources are relatively uncontrollable and typically each renewable energy source is relatively small and they are typically spread over a wide area in the power system.
The existing transmission systems are not necessarily designed to handle these new production patterns, and traditional approaches where security assessment has been carried out off-line by system planners are insufficient in today's complex networks, which was clearly seen from the major blackouts in electric power systems in Sweden and Denmark in September, 2003 and in North-Eastern and Mid-Western United States and parts of Canada in August 2003, each affecting millions of people.
Thus, because of the limited predictability of the renewable energy sources, the productions patterns may change more rapidly than before and, hence, the slow off-line calculation and/or analysis are no longer sufficient.
In response to these new production patterns, sophisticated computer tools have been developed for power system analysis and led e.g. to the use of Phasor Measurement Units (PMU's) that provide synchronized measurements in real time, of voltage and current phasors along with frequency measurements. The introduction of PMUs together with advances in computational facilities and communications, opened up for new tools for controlling, protecting, detecting and monitoring of the power systems.
Some systems have been suggested using only the system voltage phase angle measurements for assessing the system operating conditions. However, it is a disadvantage of these systems that a representation using only the voltage phase angle measurements does not provide a unique representation of a power system operating condition.
Furthermore, multidimensional nomograms have been suggested for the purpose of monitoring the overall system stability or security boundaries, however, the critical boundaries are determined in an offline analysis where multiple critical boundary points have been determined by stressing the system in various directions away from a given base operating point. However, it is a disadvantage of this approach that the boundaries are determined for a specific base case and a fixed system topology. If the system is subjected to any topological change (e.g. tripped lines due to maintenance), the actual approach may introduce an uncertainty for the assessment of security margin, as it has been based on the non-changed topological structure.
Some tools are capable of determining whether a power system is in a stable or an unstable condition, and in Jóhannsson, H., Garcia-Valle, R., Weckesser, J. T. G., Nielsen, A. H., and Østergaard, J., “Real-time stability assessment based on synchrophasors”, PowerTech, IEEE Power & Energy Society, 2011, and Jóhannsson, H. “Development of early warning methods for electric power systems”, Ph.D. Thesis, ISBN: 978-87-92465-95-5, a tool for determining the stability boundaries of the power system and the system security margins have been developed. However, while these tools may provide information on an unstable operating condition, none of the developed tools offers a remedy to bring the power system back into a stable operating condition.
Thus, there is a need for a method that can determine a remedial control action to be performed to prevent an emerging blackout and possibly bring the power system back into a stable operating condition upon experiencing instability such as a fall out of one or more power lines or generators.